Saving a rendered picture with selected Buffer?
Nope, saving the image from the Image Editor will save the "Color and Alpha" channel no matter what channel is selected.

If you want to save one of the other passes you will have to use the compositor.

The Render Layers node is the output of what blender rendered (there are lots of passes you have access to).
The Composite node will be what is displayed in the Image Editor as the Render Result.


With the two nodes connected like in the image above (Render Layers Image output to the Image input of the Composite node) you will get the exact same thing as what you could save before.



However taking the Alpha out of the Render Layers node will give you a grayscale image of just the alpha. (In most cases there is no reason to do this because if you are saving to an image that supports transparency then the alpha channel is the same as the output here.)



Now the last option in the image editor is Z depth. If you simply take the Z and plug it in to the Image output, you will get an image that looks all white. However all the values are there, they are just over 1. (If you save to an image format the supports full floats, exr is the most common, then you will have access to all the values.) But if you are saving to a png, or just want to visualize the Z data, add a Normalize node (like in the picture). That node will compress all the values to fit between 0 and 1, which is the range that your monitor can display, and a png can hold.



Now if you want to save multiple passes all at once, you can use the File Output node.


To add outputs to the file output node open up the properties region with N then click on the large "Add Input" button.